Bone resorption involves the simultaneous removal of both the mineral and the organic constituents of the extracellular matrix. This occurs mainly in an acidic phagolysosome-like extracellular compartment covered by the ruffled border of osteoclasts. Barron, et al., J. Cell Biol., 101:2210-22, (1985). Osteoclasts are multinucleate giant cells that play key roles in bone resorption. Attached to the bone surface, osteoclasts produce an acidic microenvironment between osteoclasts and bone matrix. In this acidic microenvironment, bone minerals and organic components are solubilized. Organic components, mainly type-I collagen, are thought to be solubilized by protease digestion. There is evidence that cysteine proteinases may play an important role in the degradation of organic components of bone. Among cysteine proteinases, cathepsins B, L, H, and S can degrade type-I collagen in the acidic condition. Etherington, D. J. Biochem. J., 127, 685-692 (1972). Cathepsin L is the most active of the lysosomal cysteine proteases with regard to its ability to hydrolyze azocasein, elastin, and collagen.
Cathepsins are proteases that function in the normal physiological as well as pathological degradation of connective tissue. Cathepsins play a major role in intracellular protein degradation and turnover, bone remodeling, and prohormone activation. Marx, J. L., Science. 235:285-286 (1987). Cathepsin B, H, L and S are ubiquitously expressed lysosomal cysteine proteinases that belong to the papain superfamily. They are found at constitutive levels in many tissues in the human including kidney, liver, lung and spleen. Some pathological roles of cathepsins include an involvement in glomerulonephritis, arthritis, and cancer metastasis. Sloan, B. F., and Honn, K. V., Cancer Metastasis Rev., 3:249-263 (1984). Greatly elevated levels of cathepsin L and B mRNA and protein are seen in tumor cells. Cathepsin L mRNA is also induced in fibroblasts treated with tumor promoting agents and growth factors. Kane, S. E. and Gottesman, M. M. Cancer Biology, 1:127-136 (1990).
The gene expression and cellular content of a non-cysteine protease, cathepsin D, in Alzheimer's disease brain showed evidence for early up-regulation of the endosomallysosomal system. Cataldo A. M., et al., Neuron, 1995, 14 (3), 671-680).
In vitro studies on bone resorption have shown that cathepsins L and B may be involved in the remodelling of this tissue. These lysosomal cysteine proteases digest extracellular matrix proteins such as elastin, laminin, and type I collagen under acidic conditions. Osteoclast cells require this activity to degrade the organic matrix prior to bone regeneration accomplished by osteoblasts. Several natural and synthetic inhibitors of cysteine proteinases have been effective in inhibiting the degradation of this matrix.
The isolation of cathepsins and their role in bone resorption has been the subject of an intensive study. OC-2 has recently been isolated from pure osteoclasts from rabbit bones. The OC-2 was found to encode a possible cysteine proteinase structurally related to cathepsins L and S. Tezuka, K., et al., J. Biol. Chem., 269:1106-1109, (1994).
An inhibitor of cysteine proteinases and collagenase, Z-Phe-Ala-CHN.sub.2 has been studied for its effect on the resorptive activity of isolated osteoclasts and has been found to inhibit resorption pits in dentine. Delaisse, J. M. et al., Bone, 8:305-313 (1987). Also, the effect of human recombinant cystatin C, a cysteine proteinase inhibitor, on bone resorption in vitro has been evaluated, and has been shown to significantly inhibit bone resorption which has been stimulated by parathyroid hormone. Lerner, U. H. and Grubb Anders, Journal of Bone and Mineral Research, 7:433-439, (1989). Further, a cDNA clone encoding the human cysteine protease cathepsin L has been recombinantly manufactured and expressed at high levels in E. coli in a T7 expression system. Recombinant human procathepsin L was successfully expressed at high levels and purified as both procathepsin L and active processed cathepsin L forms. Information about the possible function of the propeptide in cathepsin L folding and/or processing and about the necessity for the light chain of the enzyme for protease activity was obtained by expressing and purifying mutant enzymes carrying structural alterations in these regions. Smith, S. M. and Gottesman, M. M., J. Bio Chem., 264:20487-20495, (1989). There has also been reported the expression of a functional human cathepsin S in Saccharomyces cerevisiae and the characterization of the recombinant enzyme. Bromme, D. et al., J. Biol. Chem., 268:4832-4838 (1993).